The present invention relates to a process for producing acetic acid. More particularly, it relates to a process for producing acetic acid by the reaction of methyl formate as a starting material under pressure of carbon monoxide. Acetic acid is a basic chemical which is industrially used as starting materials for acetic esters, acetic anhydride, vinyl acetate and terephthalic acid.
Oxidation of acetaldehyde, oxidation of naphtha or olefins, and, furthermore, preparation from methanol and carbon monoxide are known as processes for the industrial production of acetic acid. As the process of reacting methanol with carbon monoxide, there are widely known processes, e.g., so-called high-pressure carbonyl process using cobalt catalysts and so-called Monsanto process carried out under low pressures using rhodium catalysts. However, according to these conventional techniques comprising carbonylation of methanol, high purity carbon monoxide is practically required in an amount more than the amount required for acetic acid to be produced. Monsanto process has a defect that it requires plant cost for generation of carbon monoxide nearly equal to plant cost for carbonylation step.
In the production of acetic acid by carbonylation reaction of methanol, coexistence of water is necessary, and even if water concentration in the reaction mixture is reduced to not more than 5% by weight as shown in the improved process, the water gas shift reaction which produces carbon dioxide and hydrogen from carbon monoxide and water cannot be completely inhibited, and, besides, for obtaining product acetic acid, much energy is needed for separation of water in the distillation step. Further defect is that a perfect mixing tank type reactor having a stirrer is used in the carbonylation reaction of methanol for the acceleration of dissolution of carbon monoxide into a liquid phase catalyst, and, hence, power for stirring is necessary.
Reaction mechanism of the carbonylation reaction of methanol is considered as shown below. As can be seen from the equation (3) mentioned below, it is a reaction consuming carbon monoxide. EQU CH.sub.3 OH+CH.sub.3 COOH.fwdarw.CH.sub.3 COOCH.sub.3 +H.sub.2 O(1) EQU CH.sub.3 COOCH.sub.3 +HI.fwdarw.CH.sub.3 I+CH.sub.3 COOH (2) EQU CH.sub.3 I+CO.fwdarw.CH.sub.3 COI (3) EQU CH.sub.3 COI+H.sub.2 O.fwdarw.CH.sub.3 COOH+HI (4)
On the other hand, for the production of acetic acid by heating methyl formate under pressure of carbon monoxide, there are known a process which uses rhodium as catalyst (JP-A-49-3513) and a process which uses a homogeneous catalyst system comprising rhodium metal atom and a mixture of lithium iodide and methyl iodide (JP-A-60-149542).
Reaction mechanism of this reaction is considered as follows. EQU HCOOCH.sub.3 +CH.sub.3 COOH.fwdarw.CH.sub.3 COOCH.sub.3 +HCOOH(5) EQU CH.sub.3 COOCH.sub.3 +HI.fwdarw.CH.sub.3 COOH+CH.sub.3 I (6) EQU CH.sub.3 I+CO.fwdarw.CH.sub.3 COI (7) EQU CH.sub.3 COI+HCOOH.fwdarw.CH.sub.3 COOH+CO+HI (8)
It can be seen from the equations (7) and (8) that being different from the process of carbonylation of methanol, consumption amount and production amount of carbon monoxide are the same when methyl formate is used as a starting material. Since the reaction having such features is utilized, in the process for the production of acetic acid from methyl formate, the apparatus for generating carbon monoxide can be of very small-scale for maintaining the carbon monoxide atmosphere in the reaction system.
The inventors found a process for producing acetic acid using methyl formate as a starting material which is released from the restriction on equipment for producing high purity carbon monoxide and, besides, minimizes the energy cost required for purification of product acetic acid and thus filed a patent application thereon (JP-Appln. 8-270855). According to this process, it has become possible to sufficiently increase space time yield and conversion which have hitherto been problems to be solved and minimized by-production of formic acid and methyl acetate which are convertible to methyl formate and acetic acid by altering reactors and reaction forms to those suitable for isomerization reaction of methyl formate. This invention can provide a process for the production of acetic acid which is economical over the Monsanto process and the improved Monsanto process.
However, though some processes for producing acetic acid using only methyl formate as a starting material have been reported by the inventors and others, there have been no examples of constructing a process which includes separation of the produced acetic acid and recycling of reaction mixture and catalyst. It has been considered that since there are many similarities between the reaction mechanism of production of acetic acid by carbonylation of methanol and the reaction mechanism of producing acetic acid by isomerization of methyl formate, carbonylation of methanol and isomerization of methyl formate are similar also in the production processes thereof, but it has not been elucidated that there are problems peculiar to the isomerization process of methyl formate.
For the purpose of establishment of a process, the inventors have conducted investigation using apparatuses which are for flash distillation and recycling of reaction mixtures, in addition to reactors. As a result, it has become clear that in some cases, decomposition of methyl formate which is a starting material and formic acid which is a reaction intermediate takes place in the flash distillation zone and the recycling zone, respectively. Unless decomposition of these compounds are inhibited, not only the features of the process for producing acetic acid by isomerization of methyl formate are lost, but also sometimes the process per se cannot be materialized.